In laser surgical treatment procedures an area to be treated is often irradiated with laser energy in a series of overlapping pulses rather than with a single pulse covering the entire area. This method can provide more accurate and safer dosage, and is also effective when dosage must vary across the treatment area.
In laser ophthalmic surgery systems, in particular those in which a uv-laser beam is used for corneal curvature modification by laser photoablation of corneal tissue, it can be useful to monitor the laser beam. This can be done, for example, for controlling the laser beam or for estimating the extent of photoablation. In early laser ophthalmic surgery systems, treatment pulses were typically centered in the area being treated, variable dosage being obtained by systematically changing the area irradiated by overlapping pulses. Monitoring of laser energy delivered to the target area was restricted to a measurement of the fluence profile of the treatment laser beam.
By way of example, U.S. Pat. No. 4,911,711, issued Mar. 27, 1990, to Telfair et al., discloses a uv-laser photoablation system which includes such a laser beam monitoring device. An operative uv-laser beam of the system is expanded to a relatively large area which terminates with a fixed cornea impingement axis, aligned with the axis of a patient's eye. The intensity profile of the laser beam is one system parameter which is selected to effect a particular corneal curvature modification.
The laser beam monitoring device includes a video camera sensitive to UV radiation. The video camera is coupled with suitable electronic circuitry to monitor the fluence profile of the operative uv-laser beam during a corneal curvature modifying operation. This is done to monitor changes in the laser beam fluence profile. The monitored laser beam fluence profile is used to control optical arrangements for further shaping that profile to ensure that the further-shaped profile stays stable during the operation.
More recent uv-photoablation systems for corneal curvature modification employ pulsed uv-lasers and arrangements for delivering a sequence or succession thousands of relatively low energy pulses in an overlapping pattern over an area of the cornea to be modified. The area of ablation spots created by these pulses is a relatively small fraction of the total area to be treated, for example, five percent or less, and is not centered therein. The extent to which corneal tissue is photoablated at a given site is determined by the total energy that impinges on that site. Examples of such systems are disclosed in U.S. Pat. No. 5,599,340, issued Feb. 4, 1997 to Simon et al. and U.S. Pat. No. 5,520,679, issued May 28, 1996, to Lin.
These types of system present a unique problem in monitoring laser-radiation fluence deposition, as it is the distribution of cumulative energy, on the cornea that determines curvature modification, rather than the energy distribution in the ablating laser beam. In such systems, in fact, the overlapping-pulse pattern compensates for non-uniformity and temporal variations in the energy distribution of the laser beam. An additional problem is presented if the monitored energy distribution is to be relied on as an accurate estimation of a photoablation profile. This is because photoablation is not linearly dependent on absorbed energy. In order to achieve a reasonably accurate estimate of a photoablation profile from an energy distribution measurement, this non-linearity must be taken into account.
It is believed that these problems of cumulative laser-beam energy distribution monitoring for estimation of photoablation have not been addressed in the prior art. A more detailed discussion of these problems, and effective solutions therefor, are presented in the description of the present invention set forth below.